Several publications and patent documents are referenced in this application in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications and documents is incorporated by reference herein.
Th17 cells, the T helper cells that produce IL-17 and other pro-inflammatory cytokines, have been shown to have key functions in a wide variety of autoimmune disease models in mice and are thought to be similarly involved in human disease (reviewed 1-3). In healthy humans, IL-17-secreting cells are present in the CD45RO+CCR6+ populations of T cells from peripheral blood 4, 5 and gut 5. Th17 cells or their products have been associated with the pathology of multiple inflammatory or autoimmune disorders in humans. IL-17 protein and Th17 CD4+ T cells are found in lesions from multiple sclerosis patients 6-8 where they are thought to contribute to the disruption of the blood-brain barrier 9. IL-17 is produced by CD4+ T cells of rheumatoid synovium 10 and is thought to contribute to inflammation in rheumatoid arthritis 11, 12. In psoriasis, products associated with Th17 cells, including IL-17, IL-17F, IL-22, and CCR6 are induced 13-15. IL-17 is induced in the gut mucosa from Crohn's disease and ulcerative colitis patients and Th17 cells are detected 13, 16. IL-23, which is produced by dendritic cells in the intestine 17, contributes significantly to Th17 cell differentiation 18. Strikingly, polymorphisms in the IL23R gene are associated with Crohn's disease, further implicating the Th17 cell pathway in the pathogenesis of this disease 19.
The mechanisms leading to differentiation of Th17 cells have been well established in mice but they are still poorly understood in humans. In mice, differentiation of Th17 cells that secrete IL-17 (also referred to as IL-17A) and IL-17F requires the expression of the transcription factors Rorγt, an orphan nuclear hormone receptor, STAT3 and IRF4 (reviewed in 20). Rorγt is sufficient to direct expression of IL-17 in activated mouse T cells 21 and is thus considered to be the effector transcription factor that establishes the Th17 differentiation lineage. Conditions that induce Th17 cell differentiation from naive murine T cells, including expression of Rorγt, have been established. Combinations of TGF-β and IL-6 22-24 or TGF-β and IL-21 25-27 are sufficient to initiate IL-17 and IL-17F expression. Expression of IL-22, considered to be another Th17 cytokine, is induced by IL-6 and inhibited by high concentrations of TGF-β 14. IL-23 is required in vivo for the generation of pathogenic Th17 cells, but it is not required in vitro for the induction of IL-17, IL-17F or IL-22 18.
In contrast to murine T cells, human T cells with a naive surface phenotype fail to produce IL-17 in the presence of TGF-β and IL-6 28-31. Increased expression of IL-17 was, however, observed by some groups in response to IL-1β alone 29 or with IL-23 15. Others have failed to observe such a response 30. These disparate findings reveal that the identities of the exogenous factors required to induce the differentiation of human Th17 cells remain unknown. The difference between the requirements for mouse and human Th17 cell differentiation have been ascribed to divergent differentiation processes, although it remains possible that T cells purified from adult peripheral blood on the basis of CD45RA expression alone are not equivalent to naive murine T cells 32-34.